1. Field of Invention
This invention relates to magnetic data recording apparatus and more particularly to improvements in such apparatus for increasing data packing density and enhancing signal to noise ratio.
2. Description of Prior Art
Electronic data processing systems make use of magnetic recording media such as disk storage devices for mass storage of data. In such devices the data is typically recorded along a plurality of parallel tracks in the form of a sequence of alternate oppositely poled magnetic flux patterns where the successive flux transitions from one polarity to the other are representative of the recorded data. To increase the storage capacity the data packing density is generally made as great as possible along each track but, as has been known for quite some time, there is a practical limit to the packing density because of pulse crowding effects. Pulse crowding is manifested in the course of reading the recorded data by the occurrence of interference between adjacent read pulses corresponding to flux transitions which are too closely spaced on the recording medium. The interpulse interference caused by pulse crowding has the deleterious effect of shifting the peaks of the read pulses, so called peak shift, and distorting the read pulse amplitude with resultant degradation of data retrieval as is well known and understood by those skilled in the art.
Various data encoding techniques have been developed heretofore for reducing the number of flux transitions per data bit in order to alleviate pulse crowding and the attendant peak shift and amplitude distortion. Also, various write compensation and read and write signal equalization techniques have been developed for use either alone or in combination with the encoding techniques for further mitigating the pulse crowding. Write equalization, for example, to which the present invention relates, provides for recording the data in such a way that the individual read pulses are narrowed and/or made more symmetrical to some degree so as to decrease interpulse interference and thereby reduce peak shift and amplitude distortion. U.S. Pat. No. 3,503,059 Pulse Crowding Compensation for Magnetic Recording, issued Mar. 24, 1970 in the name of L. E. Ambrico and U.S. Pat. application Ser. No. 893,103 An Improved Magnetic Recording Apparatus and Method, filed Apr. 3, 1978 in the name of D. T. Best are illustrative of prior art write equalization techniques related to the present invention. The Ambrico patent describes apparatus which provides for recording a minor (equalizer) flux transition opposite in polarity to and following each major (data) transition. In the absence of the minor transition, a read pulse produced from the recorded major transition tends to be asymmetrical with a trailing portion considerably longer than its leading portion as a consequence of inherent characteristics of the magnetic recording and reading process. The minor transition acts to reduce the length of the trailing portion of the read pulse. More specifically, each minor transition has an amplitude and spacing relative to its associated major transition so as to produce a pulse which overlaps and is opposite in polarity to the trailing portion of the major transition read pulse and thus adds algebraically therewith to reduce the length of the trailing portion thereof whereby the data read pulse is narrowed somewhat and made more symmetrical. The Best patent application describes apparatus which utilizes a pair of minor transitions, one preceding and the other following each major transition, for still further narrowing and improving the symmetry of the read pulses, the amplitude of the minor transitions relative to one another being unequal and their spacing relative to the major transition being asymmetrical as required to obtain appropriate algebraic combination of all the related read pulses to produce a desired composite data read pulse. Nevertheless, the system is still not optimized.
In addition to the foregoing write equalization techniques, it is also known in the prior art, as exemplified by U.S. Pat. application Ser. No. 836,820, filed Sept. 26, 1977 in the name of G. V. Jacoby et al., and now abandoned, that improvement of read data pulse symmetry can be achieved in magnetic data storage systems by combining an ac bias signal with the data transitions during the recording process. This ac bias recording technique provides excellent pulse symmetry but only slight narrowing of the data read pulses.